I was running mine at 60 front and 65 rear and they were like a brick. O went down to 50 front 45 rear and it was an improvement. Like I said with 20s and the low sidewall you have you probably won't notice much difference but I'd try 50 and 45. Before you go crazy, you may want to try to get a ride in a stock 2500. I can't tell you how often I end up cursing on bumpy roads here in Pennsylvania. You may be expecting too much in terms of ride quality in a lifted 2500. A stock truck will let you know what the best ride that could be expected.
As far as your lift, I'm guessing it's a 6 inch. Typically the fronts of these trucks are 3 inches higher than the rear. If you measure the block under your rear springs, I'd suspect from the picture that it's 3 inches high. So to level that truck with a 3 inch block in the rear, you'd need a 6 inch spring on the front making it a 6 inch kit. So if u buy shocks you need shocks for a 6 inch lifted truck. If I were going to replace my shocks, I'd try the Fox personally. I've never used them, but like I said I wasn't impressed with the Bilsteins I tried. The Fox get pretty good reviews. As @DDP stated Carli and Fox are top sellers. He can probably chime in with determining exactly what length you need, but honestly at the end of the day, I think you'll find that there's not going to be too much you can really do to improve the ride quality without softer springs. And even that I'd guess will be a marginal improvement with a pretty big investment. I've been driving lifted trucks and Jeeps all my life. Now at 50 there is just too much downside for me to have a lifted daily driver. I sure love the look but a leveled 2500 is no small truck to begin with and keeps me happy. I still consider lifting mine at least once a month, but then I talk myself off the ledge lol!
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